On the effective conductivity of flat random two-phase models

An approximate equation for the effective conductivity sigma_eff of systems with a finite maximal scale of inhomogeneities is deduced. An exact solution of this equation is found and its physical meaning is discussed. A two-phase randomly inhomogeneous model is constructed by a hierarchical method and its effective conductivity at arbitrary phase concentrations is found in the mean-field-like approximation. These expressions satisfy all the necessary symmetries, reproduce the known formulas for sigma_eff in the weakly inhomogeneous case and coincide with two recently found partial solutions of the duality relation. It means that sigma_eff even of two-phase randomly inhomogeneous system may be a nonuniversal function and can depend on some details of the structure of the inhomogeneous regions. The percolation limit is briefly discussed.Comment: 8 pages, 2 figures, Latex2

Kink-Antikink Unbinding Transition in the Two Dimensional Fully Frustrated XY Model

We carry out the first numerical simulations to directly confirm the existence of a kink-antikink unbinding transition, T_w, along Ising-like domain walls in the two dimensional fully frustrated XY model. We comment on the possible implications of kink-antikink unbinding for the bulk phase transitions of the model.Comment: 8 pages, 10 figures - expanded version of original submission as accepted by Phys. Rev. B. Some correction to the analysis lead to an increase in the value of the kink unbinding transition temperatur